This invention relates to a high voltage composite member such as a support insulator or a bushing and to a method of making the same.
More particularly, a composite member here of concern is of the type including a central part which, in the case of a support insulator, is a fiberglass reinforced thermosetting plastic rod and which, in the case of a bushing, is a bare conductive rod or tube or a paper-wound conductive rod or tube, with metal foil at appropriate spacings within the paper layers, the whole being impregnated with epoxy to form a solid mass when the epoxy cures. In any case, an elastomeric weathershed housing surrounds the central part. Such a support insulator is then terminated by suitable metal end fittings and such a bushing is typically provided with a mounting flange.
Electrical transmission lines are typically supported by insulators mounted on poles. For many years, porcelain or glass insulators were used for this purpose, but because of certain drawbacks, such as excessive weight and relative fragility, particularly in applications where gunfire vandalism is a problem, procelain and glass insulators are being supplanted by composite insulators as described in the immediately preceding paragraph.
Kalb U.S. Pat. No. 3,898,372, issued Aug. 5, 1975 discloses one of the simplest composite electrical insulators, such comprising a series of separately molded modules each providing a single shed. The modules are compressed between metallic end-fittings on a fiberglass rod. The central aperture of each module has a diameter smaller than the rod diameter. The modules are under hoop tension as a consequence of both the compression between the end-fittings and the diameter difference between the shed aperture and the rod. Any voids between the rod and the modules are filled with a pasty dielectric grease-like material.
Ishihara et al. U.S. Pat. No. 4,296,276, issued Oct. 20, 1981, discloses a composite insulator comprising a plastic rod, a single module having two or more sheds, a pasty dielectric between the rod and the module, and metal end fittings. To reduce hoop tension due to compression of the module between the end fittings, the latter are provided with metal sleeves which are pressed onto the end portions of the module, so that those end portions are sandwiched between the metal sleeves and the plastic rod. The Ishihara et al. patent discloses that the elongation of the outer surface of the module resulting from assembly of the insulator is not higher than 2%, as distinguished from 5% or more in the device of the Kalb patent.
I have found that over extended periods of time, residual elongation of the module may lead to eventual splitting of the elastomer, even when the elongation is as little as 1.5% or less, particularly when other forms of energy such as high electrical stress, high leakage current or even ultraviolet light from the sun are present.
Lusk et al, U.S. Pat. No. 4,212,696, issued July 15, 1980, discloses a variation on the designs discussed above, involving replacement of the pasty dielectric grease-like material of the Kalb patent by an adhesive, but the major drawback, i.e., residual shel elongation, remains.
An alternative prior art design consists of an elastomer weathershed molded directly onto a central core. Such design has been commercially available for many years for distribution voltage insulators. Those insulators are relatively short, generally not exceeding two feet or so in length and can therefore be readily and economically molded by known means in conventional rubber molding presses. However, molded designs become more difficult to produce when the insulator is of a length typical of transmission voltage insulators, which generally exceed three feet in length and frequently are up to ten feet or more in length. For economy, it is desirable to have single piece transmission voltage composite insulators and not a chain of shorter insulators which require numerous expensive metal connectors to tie the chain together. Furthermore, sheds which are molded directly to a central core fiberglass rod do not readily permit undercuts on the sheds, due to difficulty in removing such shapes from inexpensive two-part molds. Undercuts are desirable to provide rain drip points on vertically mounted insulators, to limit leakage current under wet conditions. More complex and expensive multi-part molds are therefore required in order to provide undercuts. Molded distribution composite insulators are normally used in a horizontal mounting, so undercuts are not a requirement for those product sizes.
A superior and more readily produced design uses a sheath extruded directly onto the central core and subsequently vulcanized by known means to the central core. Such a design is described in my U.S. Pat. No. 4,312,123, issued Jan. 26, 1982, now assigned to the assignee hereof. The sheds in that design are provided by modules which are molded separately with a central aperture slightly less in diameter than that of the extruded sheathed core. They are then expanded and placed over the sheath prior to vulcanization of the sheath. Subsequent contraction of the module to the diameter of the sheath (but not to the module's original aperture diameter) provides a firm compression grip of the module to the sheathed core during assembly and vulcanization of the sheath. However, during vulcanization the sheath becomes softened and, due to the clamping pressure produced by the still remaining hoop tension of the expanded module, the sheath flows out slightly from under the module, if the modules do not abut one another nor completely cover the sheath along its length. This flowing out eliminates any residual expansion or elongation of the modules. During vulcanization the sheath will bond to the rod and to the modules, as is well known in elastomer curing technology. Furthermore, there are other significant benefits gained by a design in which the sheath is not entirely covered by the modules. Such benefits include the ability to alter the leakage distance per unit insulator length by spacing the modules more closely or less closely to one another along the length of the sheath. It is, of course, necessary that in such a design the sheath be as resistant to the effects of weather exposure as are the modules. This resistance can be achieved by using a similar composition for both the sheath and the individual modules. Another benefit of the design is that it eliminates problems created by the narrow crevices between abutting sheds that have been associated with such designs and are generally attributed to leakage current effects at such crevices.
This design of separately molded, spaced apart modules over a sheathed core vulcanized to form an integral structure can experience difficulties if the modules do not bond adequately to the sheath during the vulcanization of the sheath. Precautions must be taken to have clean surfaces of both sheath and module in intimate contact throughout the vulcanization step. Although the intimate contact may be achieved by diameter differences between module aperture and sheath as previously described, adequate cleanliness and readily bonded surfaces are less readily achieved consistently. In the usual factory conditions, normal handling can result in deposits from the hands of the workers on the sheath or sheds which may result in poor bonding. Oily deposits or perspiration are particularly serious in this respect.
Additionally, certain ingredients of the compounded rubber which are desirable for other reasons may bloom to the surface during storage of the unassembled components or during processing of the insulator assembly. These ingredients may create a surface condition which is not conducive to adequate bonding during vulcanization. Should voids result between module and sheath, various well known problems of leakage currents, water penetration and possible corona discharges may occur through or in the voids, all of which are deleterious to the insulator performing its function over an extended period of time.
The present invention presents several advantages over previous art, while retaining all of the benefits of my U.S. Pat. No. 4,312,123.
Among these advantages of the present invention are the elimination of the need for vulcanization bonding of the module to the sheath and the consequent enablement of the use of other elastomeric materials for the modules, which materials may be difficult or impossible to bond adequately to the sheath during vulcanization. Particularly useful in this regard are thermoplastic elastomers. Attendant to their use is substantial saving in scrap loss, since thermoplastic materials may be recycled, which is not possible with vulcanized thermosetting elastomeric materials.
Other objects and advantages will appear hereinafter.